In oil and gas exploration and production it is common to use a down hole motor which is driven by an flowing incompressible fluid to rotate an attached drill bit. The fluid is often, but not necessarily, high specific gravity fluid such as drilling mud. The mud (or other incompressible fluid) can also act to clear cuttings from the hole and provide down hole pressure control. Additionally it is sometimes possible to increase the volumetric flow rate of mud through a down hole motor to kill a well if required.
However there is a limitation in terms of drilling in hard materials particularly with directional (i.e. non-vertical holes). This arises due to the inability to apply sufficient down hole pull-down or weight on bit (“WOB”) on the drill bit to fracture rock and progress the drilling at an economic rate.
The limitation of penetration in hard materials can be overcome by the use of a hammer drill. Hammer drills are driven by a fluid. Air is a common driving fluid. However air does not enable control of down hole and ground pressure. Also it is often not possible to provide the air with the required pressure and volume to provide sufficient pressure differential with reference to the prevailing down hole environment to effectively drive the hammer.
Instead of air, water and additives such as drilling mud can used to drive the hammer. This enables higher drilling pressures to be provided to combat high ground pressures. However due to its inherent nature the mud rapidly wears the internal surfaces of the hammer leading to the need for frequent replacement. This involves the very time consuming process of tripping the drill string. Also conventional hammer drills do not enable a sufficient volumetric flow rate to kill a well (i.e. flood the well quickly to control or stop the flow of gas and other dangerous well conditions) in the event of a dangerous over pressure condition.